This invention relates to a hydrometallurgical process for recovering iron and zinc from the baghouse dust that is generated in steel production and other metallurgical operations.
Baghouse dust is a mixture of metal oxides that are collected by scrubbers or electrostatic precipitators in electric arc furnace (EAF) and blast furnace steel-making facilities and in other iron-making plants. According to recent estimates, as cited by W. D. Huskonen in Metal Producing, 33.38-39 (October 1989), American EAF steel makers generate about 500,000 tons/year of toxic dust, whose dumping in landfills was to be banned by the U.S. Environmental Protection Agency as of Aug. 8, 1990. This dust contains leachable forms of lead, cadmium, and chromium, whose runoff may enter the groundwater system and contaminate drinking water. It also contains 30-50 weight-% of iron and, in carbon steel production, about 5-50 weight-% of zinc. The amount of zinc in EAF dust is expected to grow due to a) a projected doubling in the amount of zinc that will be used on flat-rolled steel and b) a recent introduction of new galvanizing lines that should cause a four-fold increase in the amount of galvanized scrap to be remelted.
"Upgrading" of a portion of the dust by raising its zinc content to &gt;50 weight-% would yield a product that could be sold as a raw material to refiners of metallic zinc. However, a large proportion of the zinc in the dust is combined with iron in the form of zinc ferrite, ZnFe.sub.2 O.sub.4, which cannot be easily decomposed into separate iron and zinc constituents.
It is an object of this invention to provide an energy-efficient and cost-effective process for separating zinc from iron in baghouse dust.
The approaches that have been reportedly considered thus far for recycling or disposing of baghouse dust fall into the following three general categories:
1. Briquetting, pelletizing or otherwise fixating the dust in a leachproof matrix and storing or disposing of the fixated product
2. Reducing the dust with coal, methane or hydrogen at an elevated temperature and separating condensable zinc vapor from a nonvolatile slag, e.g., using a plasma furnace or a flame reactor process.
3. Removing the zinc by a hydrometallurgical process.
The first of these approaches involves inputs of energy, materials, labor, and operating facilities to generate a disposable but nonsellable product. The second approach yields reusable zinc and iron, but necessitates costly, rather large, thermally insulated facilities and considerable energy expenditures. The hydrometallurgical approaches should involve the least costly equipment and the least expenditures of energy, but they may generate excessive amounts of environmentally objectionable chemical effluents.
More particularly, the hydrometallurgical processes that are widely used for recovering zinc from iron-containing ores are based on sequences that include selective dissolution (leaching), precipitation, filtration, and washing. These processes yield high recoveries (about 98%) of zinc, iron, lead, and other ore constituents. However, they generate a voluminous environmentally objectionable waste stream. For instance, one of the most widely used of these processes--the so-called "jarosite process"--comprises leaching the ore with hot dilute sulfuric acid to yield dissolved ferric and zinc sulfates followed by neutralization of the acid to precipitate the iron in form of jarosite. These steps generate a large volume of environmentally objectionable chemical waste.
It is therefore another object of this invention to provide a hydrometallurgical process for recovering zinc and iron from baghouse dust that will offer the advantages of relatively low costs of equipment, energy, and materials, and yet generate no objectionable chemical waste stream.
Other objects of the invention will become apparent to professionals in the environmental, metallurgical, and related areas following perusal of the complete specification.